This invention relates to transmit diversity in telecommunication systems and, more particularly, to a method and apparatus for transmitting on adaptive antenna transmit arrays using a reduced number of CDMA pilot channels.
As wireless communication systems evolve, wireless system design has become increasingly demanding in relation to equipment and performance requirements. Future wireless systems, which will be third and fourth generation systems compared to the first generation analog and second generation digital systems currently in use, will be-required to provide high quality high transmission rate data services in addition to high quality voice services. Concurrent with these system service performance requirements will be equipment design constraints, which will strongly impact the design of mobile stations. The third and fourth generation wireless mobile stations will be required to be smaller, lighter, and more power-efficient units that are also capable of providing the sophisticated voice and data services of these future wireless systems.
Time-varying multi-path fading is an effect in wireless systems, whereby a transmitted signal propagates along multiple paths to a receiver causing fading of the received signal due to the constructive and destructive summing of the signals at the receiver. Several methods are known for overcoming the effects of multi-path fading, such as time interleaving with error-correction coding, implementing frequency diversity by utilizing spread spectrum techniques, or transmitter power control techniques. Each of these techniques, however, has drawbacks with regard to use for third and fourth generation wireless systems. Time interleaving may introduce unnecessary delay, spread spectrum techniques may require large bandwidth allocation to overcome a large coherence bandwidth, and power control techniques may require higher transmitter power than is desirable or sophisticated receiver-to-transmitter feedback techniques that increase mobile station (MS) complexity. All these drawbacks have negative impact on achieving the desired characteristics for third and fourth generation mobile stations.
Antenna diversity is another technique for overcoming the effects of multi-path fading in wireless systems. In diversity reception, two or more physically separated antennas are used to receive a signal, which is then processed through combining and switching to generate a received signal. A drawback of diversity reception is that the physical separation required between antennas may make diversity reception impractical for use on the forward link in the new wireless systems where small MS size is desired. A second technique for implementing antenna diversity is transmit diversity. In transmit diversity, a signal is transmitted from two or more antennas and then processed at the receiver by using maximum likelihood sequence estimator (MLSE) or minimum mean square error (MMSE) techniques. Transmit diversity has more practical application to the forward link in wireless systems in that it is easier to implement multiple antennas in the base station than in the MS.
One method of transmit diversity, called the Switched Transmit Diversity, proposes transmission using the best antenna at any given instant. Another method, called Orthogonal Transmit Diversity (OTD), splits the data stream into multiple streams and transmits the data using orthogonal CDMA codes.
Transmit diversity techniques have been shown to provide advantages over single-antenna systems in CDMA forward link transmission. Transmit diversity using open-loop and closed-loop methods have been considered, and closed-loop methods in general have been shown to be preferred. In open-loop methods, transmitter parameters are adjusted without feedback from the receiver. This may involve, for example, adjusting transmitter parameters on the forward link based on measurements made on signals received on the reverse link. In closed-loop methods, feedback information from the receiver is used to adjust transmitter parameters. Several closed-loop methods for transmit diversity have been proposed, such as space time diversity, orthogonal transmit diversity (OTD), and time space transmit diversity (TSTD) and transmit adaptive arrays (TX AA). In TX AA, transmission parameters of multiple antennas are weighted in such a way that the power received by the mobile is maximized. For a single path channel, the optimal weights for the antennas are the conjugates of the respective channel coefficients. For multipath channel conditions, each antenna would optimally have multiple filter tap weights. It has been shown that using one tap weight per antenna even in this case provides better performance than the single antenna configuration. Implementation of TX AA is a trade-off that results in a reduction of reverse link capacity in order to facilitate an increase in forward link capacity. TX AA using two transmit antennas requires three forward link pilots: (1) a broadcast common pilot (Pilot0,), (2) a broadcast auxiliary pilot (Aux Pilot1) for the second antenna, and (3) a dedicated auxiliary pilot (Aux Pilotu) for each user.
In TX AA with a dedicated auxiliary pilot, Aux Pilotu, for each user, the traffic channels (TCHs), along with each user auxiliary pilot, Aux Pilotu, are transmitted through the two antennas after appropriate weighting. The TX AA weights in the ideal case are the conjugates of the channel coefficients h0(t) and h1(t). These channel coefficients are estimated by the MS by using matched filters on the received antenna pilot signals (Pilot0 and Aux Pilot1). A quantized ratio of the channel coefficients is fed back to the base station. This ratio is a complex number, with gain and phase information. The phase is allotted a larger number of bits of quantization than the gain. Some modes of TX AA use a phase-only feedback method.
Both the traffic channel signal and the user specific auxiliary pilot, Aux Pilotu, are altered by the TX AA weights and also the transmission channel before they are received at the MS. For coherent demodulation of the TCH, information on both the TX AA weights and transmission channel are necessary for each antenna path. The TX AA weights are actually calculated at the MS, so ideally that information should be available to the MS. In reality, however, there is a bit error rate associated with the feedback, which means that the TX AA weighting applied at the base station is not always the same as that calculated at the MS. The Aux Pilotu is thus utilized in order to obtain an estimate of the exact gain and phase change that was undergone by the TCH, since Aux Pilotu undergoes the same changes. Thus, for maximal ratio combining (MRC) at the receiver, the weights are obtained by also using a matched filter on Aux Pilotu.
The invention provides an adaptive transmit antenna array having a reduced pilot set. According to the invention, information used to estimate the maximal ratio combining (MRC) weights for an adaptive transmit array is obtained through other than a user-specific pilot. The adaptive transmit antenna array utilizes a decision-directed mechanism for the estimation of the MRC weights to be used at the receiver. This allows an adaptive transmit antenna array to be implemented without requiring a user-specific pilot. Reducing the number of pilot channels in a system results in decreased overhead requirements for the system.
In an embodiment of the adaptive transmit antenna array of the invention, a first and a second signal are each transmitted on a traffic channel from a separate antenna of two antennas. The signals are each weighted by separate weights. The weighting is performed by multiplying each signal by weights that are estimates of the complex conjugate of the channel coefficients. The weights are-the complex conjugate of the channel coefficients in the case of 1-path channels, or they are extracted from the principal eigen vector of the channel correlation matrix in the case of multipath channels. The signals are received at a receiver and demodulated and decoded on a frame-by-frame basis. The signal as received, before demodulation and decoding, is also stored in the receiver. A determination is then made at the-receiver, for each frame, as to whether or not the frame has been correctly decoded. If the frame has been correctly decoded, the frame is processed further by the receiver. If it is determined that the frame has not been correctly decoded, the decoded symbols of the frame are re-encoded and remodulated. The conjugate of the remodulated symbols is then multiplied with the symbols as received on the traffic channel to estimate new weights. The resultant estimate weights are averaged over the period for which the transmit antenna array weighting is constant. These new weight estimates are then used to again demodulate and decode the frames as received on the traffic channel. If no error is detected on the received frame, as decoded using the new weight estimates, the frame is processed further in the receiver. If an error is detected, an error message for the frame is sent to the transmitter.